1. Field of the Invention
The present invention relates to an electrolyte for a lithium ion rechargeable battery and a lithium ion rechargeable battery that includes the electrolyte. In particular, the present invention provides an electrolyte for a lithium ion rechargeable battery that provides excellent cycle life characteristics and high-temperature storage stability. The electrolyte also prevents a drop in discharge capacity of a battery at low temperatures. The present invention also provides a lithium ion rechargeable battery that includes the electrolyte.
2. Description of the Prior Art
As the electronics industry has advanced, technical research into portable and wireless electronic instruments including telephones, vi1deo cameras, and personal computers has progressed rapidly. Accordingly, a small rechargeable battery that is lightweight and has high energy density is increasingly in demand to power these instruments. Particularly, a rechargeable battery that has a non-aqueous electrolyte and uses a lithium-containing metal oxide as a cathode active material and a carbonaceous material capable of lithium intercalation/deintercalation as an anode active material to provide a voltage of about 4 V fills these requirements.
Lithium ion rechargeable batteries have an average discharge voltage of about 3.6 to 3.7 V and thus can provide relatively high electric power compared to other alkali batteries, Ni—MH batteries, Ni—Cd batteries, etc. However, in order to obtain such a high drive voltage level, an electrolyte composition that is electrochemically stable in the charge/discharge voltage range of 0 to 4.2V is required. For this reason, a mixture containing a cyclic carbonate-based solvent such as ethylene carbonate, propylene carbonate, and butylene carbonate, for example, is generally used as an electrolyte.
During the initial charge cycle of a lithium ion rechargeable battery, lithium ions are discharged from a lithium metal oxide, which is the cathode active material, and move toward a carbon electrode, which is the anode, so that lithium ion intercalation into carbon can be made. During this process, the lithium may react with the carbon electrode to produce Li2CO3, Li2O, LiOH, etc., thereby forming a film on the surface of the anode. Such a film is referred to as a Solid Electrolyte Interface (SEI) film.
After the SEI film is formed at the initial charge cycle, it serves as a barrier for preventing lithium ions from reacting with the carbon anode or other substances and also forms an ion tunnel during the following charge/discharge cycles. The ion tunnel prevents collapse of the carbon anode that is caused by the dissolving lithium ions in high-molecular weight organic solvents that are present in the electrolyte. It also prevents the movement of lithium ions with the solvents, which results in intercalation into the carbon anode. Therefore, once the SEI film is formed, lithium ions are prevented from reacting with the carbon anode again or from undesirably reacting with other substances. Thus the concentration of lithium ions can be maintained constant.
However, as charge and discharge cycles repeat electrode plates repeatedly expand and shrink and local over-voltage may be applied. Under these circumstances, a passivation layer such as an SEI film may be gradually degraded with the lapse of time and the surface of the anode may be exposed and may undesirably react with the surrounding electrolyte. In addition, gases are generated from the undesired side-reaction, which thereby increases the internal pressure of the battery and significantly degrades the cycle life characteristics of a battery. The gases that are generated mainly include CO, CO2, CH4, C2H6, etc., depending on the kind of the carbonate used in the electrolyte and the type of anode active material (J. Power Sources, 72 (1998) p. 66-70).
Additionally, a certain graphite-based anode active material may cause the decomposition of a carbonate-based electrolyte and the separation of a carbonaceous material, thereby detracting from characteristics of a battery including electric capacity, cycle life characteristics and storage characteristics. Particularly, such problems are exacerbated for batteries that use an electrolyte that contains propylene carbonate. Propylene carbonate is decomposed at an anode during the first charge cycle, thereby decreasing the initial capacity significantly.
In order to prevent the decomposition of cyclic carbonates and the separation of carbonaceous materials caused by graphite-based anode active materials, a method of adding a crown ether (12-crown-4) to an electrolyte based on propylene carbonate and ethylene carbonate has been suggested (J. Electrochem. Soc., Vol. 140, No. 6, L101 (1993)). However, this method is problematic in that a large amount of expensive crown ether is needed to prevent the decomposition of cyclic carbonates to a desired degree, and the battery characteristics obtained by the method are not sufficient for practical use.
Additionally, Japanese Patent Laid-Open No. Hei 8-45545 discloses a method of adding vinylene carbonate to an electrolyte based on propylene carbonate and ethylene carbonate in order to prevent decomposition of the electrolyte. According to the method, vinylene carbonate is reduced at an anode during charge cycles to form an insoluble film on the surface of graphite (anode), thereby preventing reduction of solvents such as propylene carbonate and ethylene carbonate.
However, this method using vinylene carbonate alone cannot accomplish the formation of a complete SEI film at the first charge cycle. As charge and discharge cycles are repeated at room temperature, the film may crack and vinylene carbonate is decomposed and consumed again in order to compensate for such cracked portions. Ultimately, it is not possible to obtain stable cycle life characteristics of a battery. Further, although cycle life characteristics of a battery may improve by increasing the amount of vinylene carbonate, the method still has problems in that the discharge capacity of a battery decreases rapidly at low temperature and swelling of a battery may occur when it is stored at high temperature.